Neuronal pain pathway

ABSTRACT

The present invention relates to the discovery of a novel molecular pathway involved in long-term hyperexcitability of sensory neurons, which, in higher animals, is associated with persistent pain. It is based on the discovery that, following injury to an axon of a neuron, an increase in nitric oxide synthase activity results in increased nitric oxide production, which, in turn, activates guanylyl cyclase, thereby increasing levels of cGMP. Increased cGMP results in activation of protein kinase G (“PKG”), which then is retrogradely transported along the axon to the neuron cell body, where it phosphorylates MAPKerk.

The present application claims priority to U.S. Provisional Patent Application Ser. No. 60/713,435 which was filed Sep. 1, 2005, and U.S. Provisional Patent Application Ser. No. 60/664,071 which was filed Mar. 21, 2005 each of which is hereby incorporated by reference in their entireties.

GRANT INFORMATION

The subject matter of this application was developed at least in part under National Institutes of Health Grants NS12250 and NS35979, so that the United States Government has certain rights herein.

1. INTRODUCTION

The present invention relates to the discovery of a novel molecular pathway involved in long-term hyperexcitability of sensory neurons, which, in higher animals, is associated with persistent pain.

2. BACKGROUND OF THE INVENTION

Pain is perceived as a result of communication between the two main divisions—central and peripheral—of the nervous system. While the two divisions work together to produce our subjective experience, the central and peripheral nervous systems are anatomically and functionally different.

A painful stimulus impinging on a specialized pain receptor is propagated along a peripheral branch of a sensory axon to a neuron lying within a dorsal root ganglion (part of the peripheral nervous system) and then along a central branch of the axon into the spinal cord (central nervous system). The signal is subsequently relayed to a central nervous system neuron in the spinal cord which in turn passes the signal, through its axon, to the opposite (“contralateral”) side of the spinal cord and then up to pain perceiving structures in the brain.

Peripheral pain receptors are located on free nerve endings which can respond to mechanical, thermal or chemical stimuli. Pain can be acute or chronic. Acute pain is typically transmitted from the receptor through Aδ sensory nerve fibers, which are thinly coated with the insulating compound, myelin, which facilitates impulse conduction. Chronic pain typically travels through C fibers, which, because they are unmyelinated, transmit impulses slowly, leading to the characteristic dull, diffuse nature of chronic pain. Chemical mediators of inflammation such as bradykinin and prostaglandins stimulate pain receptors, and are important agents in chronic pain syndromes, such as the persistent pain associated with arthritis or nerve inflammation.

The perception of pain can be altered at various stages of the pain pathway. For example, the painful stimulus can be eliminated by administering a local anesthetic to the peripheral receptor. Drugs like opioids were classically known to intervene at the central nervous system stage of the pain pathway, and non-steroidal anti-inflammatory drugs at the peripheral stage (although it is now realized that there is some cross-reactivity of both). Likewise, what is perceived as chronic pain (not due to primary spinal cord injury) is typically associated with sensitization of peripheral pain receptors as well as changes in the excitability of spinal neurons, and therefore has both peripheral and central nervous system components. The peripheral and central components involved in chronic pain are referred to, respectively, as “primary” and “secondary” hyperalgesia (Urban and Gebhart, 1999, citing Woolf, 1983 and La Motte et al., 1991).

In terms of the central nervous system components of chronic pain, the spinal cord neuron which receives the stimulus from the dorsal root ganglion axon, exhibits changes in gene expression in the context of chronic pain and is believed to contribute to the phenomenon of “central sensitization” or “spinal hyperalgesia.” Spinal N-methyl-D-aspartate (“NMDA”) receptors are believed to play an important role in this process (Urban and Gebhart, 1999, citing Urban and Gebhart, 1998; Palacek et al., 2003; Lee et al., 1993). Spinal cord injury without activation of the peripheral nervous system can also produce spinal hyperalgesia resulting in a central pain syndrome (Zhang et al., 2005). Central neuropathic pain has been associated with phosphorylation of the transcription factor, cyclic AMP response element binding protein (“CREB”) (Cron et al., 2005).

Regarding the peripheral nervous system component of chronic pain associated with nerve injury (“neuropathic pain”), persistent neuropathic pain is a major clinical problem that has mostly resisted effective treatment. In humans (Gracely et al., 1992) and mammalian model systems (Millan, 1999), persistent pain after nerve injury is associated with long-term hyperexcitability (LTH) of sensory neurons (SNs) having axons in the injured nerve. LTH is manifested as increased sensitivity to electrical stimuli in the SN cell body and axon at the injury site (Wall and Devor, 1983; Study and Kral, 1996; Zhang et al., 1997; Chen and Devor, 1998; Kim et al., 1998; Abdulla and Smith, 2001). These changes result in discharge of action potentials from SNs at rest or during innocuous stimulation, leading to continuing excitation of higher order neurons in the central nervous system and to secondary, or spinal hyperalgesia and persistent pain. Because the appearance of LTH involves alterations in gene expression (Waxman et al., 1994; Wang et al., 2002; Park et al., 2003), a central question is, how are such changes in the nucleus induced by an injury that occurs far from the cell body? Answering this question has been extremely difficult using the complex mammalian nervous system.

An experimentally favorable alternative is the homogeneous cluster of SNs that reside in the bilateral pleural ganglia of the mollusk Aplysia californica (Walters et al., 2004). Noxious mechanical stimulation of the body wall (Walters et al., 1983a) or crushing SN axons in vivo or in vitro, elicits an LTH with electrophysiological properties similar to those seen after axotomy of mammalian SNs (Walters et al., 1991; Walters, 1994; Ambron et al., 1996; Bedi et al., 1998; Ungless et al., 2002; Sung and Ambron, 2004). The LTH appears after a delay, suggesting that its induction after nerve crush is attributable to a positive molecular injury signal (Walters et al., 1991; Ambron and Walters, 1996; Lin et al., 2003). Two studies support this idea. First, blocking axonal transport after nerve injury in excised nervous systems prevented the appearance of LTH (Gunstream et al., 1995). Second, LTH was induced in noninjured SNs by injecting axoplasm from injured axons (Ambron et al., 1995). LTH was also elicited in the SNs after intrasomatic injection of an ERK (extracellular signal-regulated kinase) member of the MAPK (mitogen-activated protein kinase) family (Sung et al., 2000). Other experiments have suggested that cGMP and PKG (cGMP-dependent protein kinase; protein kinase G) are probably involved (Lewin and Walters, 1999). However, despite these observations, the identity of the signal from the axon, how PKG and the ERK are activated, or how these kinases might interact were not known. Moreover, LTH was also reported to be induced by cAMP acting on PKA (protein kinase A) in a learning paradigm (Dale et al., 1988; Scholz and Byrne, 1988).

U.S. Pat. No. 6,476,007 by Tao and Johns (“Tao and Johns”) relates to a proposed signalling pathway in the central nervous system in which stimulation of an N-methyl-D-aspartate (“NMDA”) receptor leads to activation of nitric oxide synthase (“NOS”) and production of nitric oxide (“NO”), which then stimulates guanylate cylase (“GC”) and the production of cyclic guanoside monophosphate (cGMP), which in turn activates cGMP-dependent protein kinase Iα (“PKG”). It was observed that administration of the PKG inhibitor Rp-8-[4-chlorophenyl)thio-cGMPS triethylamine into the central nervous system by intrathecal administration, after the induction of an inflammatory response, produced significant antinociception in rats 10 and 60 minutes later. Further, they noted an upregulation of PKG expression in the lumbar spinal cord 96 hours after noxious stimulation was blocked by administration of a neuronal NOS inhibitor, a soluble GC inhibitor, and a NMDA receptor antagonist.

However, while Tao and Johns purports to address the mechanism of inflammatory hyperalgesia in the central nervous system, prior to the present invention the need remained to determine the mechanism of pain, and in particular chronic pain and long-term hyperexcitability, in the sensory neurons of the peripheral nervous system.

The need to address the mechanism of pain in the peripheral nervous system is important for several reasons, the first of which is drug accessibility. The central nervous system is sequestered from the rest of the body by the blood-brain-barrier, which is created by tight junctions between endothelial cells of the central nervous system and prevents many therapeutic drugs from ever reaching the central nervous system. Because of the extremely limited permeability of the blood-brain-barrier, treatment of spinal hyperalgesia according to Tao and Johns would be problematic. The ability, according to the present invention, to treat the primary hyperalgesia aspect of pain by delivering agents to the peripheral nervous system, which does not have the same permeability issues, confers a substantial advantage.

A second reason that treatment of peripheral pain mechanisms is important is that the periphery is the portal for pain perception. The present invention offers the advantage of intervening in subjective pain as it first arises, such as in the context of a normally non-painful stimulous which results in the perception of pain as a result of long term hyperexcitability (LTH). Subjective pain can be triggered in chronic pain sufferers by stimuli—such as the light touch of a sheet or a passing breeze—which would not normally be painful. The present invention is directed at this first stage of the pain pathway.

3. SUMMARY OF THE INVENTION

The present invention relates to the discovery of a novel molecular pathway involved in long-term hyperexcitability of sensory neurons in the peripheral nervous system, which, in higher animals, is associated with persistent pain. It is based on the discovery that, following injury to an axon of a sensory neuron, an increase in nitric oxide synthase (“NOS”) activity results in increased nitric oxide (“NO”) production, which, in turn, activates guanylyl cyclase (“GC”), thereby increasing levels of cyclic guanosine monophosphate (“cGMP”). Increased cGMP results in activation of protein kinase G (“PKG”), which then is retrogradely transported along the axon to the neuron cell body, where it phosphorylates mitogen-activated protein kinase-erk (“MAPKerk”). The activated MAPKerk then translocates into the cell nucleus, where it modulates expression of pain-related genes.

In various embodiments, the present invention provides for methods of and compositions for inhibiting and/or treating long-term hyperexcitability and/or persistent pain in a subject suffering from primary (peripheral nervous system) hyperalgesia, comprising administering, to the subject, an agent that inhibits a step in the above pathway. In particular embodiments, the agent is administered so as to be delivered to a sensory neuron in a dorsal root ganglion. In specific embodiments, the agent comprises a peptide that facilitates retrograde transport from the periphery to a sensory neuron cell body. In further embodiments, the present invention provides for assays that may be used to identify molecules that can modulate (inhibit or promote) steps in this pathway and thereby modulate long-term hyperexcitability and/or pain perception.

In still further embodiments, the present invention provides for a cloned PKG gene of Aplysia californica, its encoded protein and homologues thereof, and antibodies directed toward the purified protein.

4. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A-D. A, Top, A schematic diagram of apPKG showing the position of the conserved tandem cGMP binding domains, the ATP binding and catalytic sites, and the position of an autoinhibitory sequence. Bottom, Clustal W sequence alignment of the predicted apPKG amino acid sequence with Drosophila DG1 (GenBank accession number AAB03405; SEQ ID NO:52) and DG2T3a (AAA28459; SEQ ID NO:53), human Iα (BAA08297; SEQ ID NO:54) and II (CAA64318; SEQ ID NO:55), mouse Iβ (AAD16044; SEQ ID NO:56) and II (AAA02572; SEQ ID NO:57), and rat II (CAA85284; SEQ ID NO:58) PKGs. Conserved amino acids are shaded in black; similar amino acids are shaded in light gray. B, Phylogenetic analysis of the PKG family. C, Expression of apPKG in neurons. A multiple-tissue Northern blot was hybridized with a ³²P-labeled N-terminal apPKG cDNA fragment. The arrow indicates apPKG mRNA. The sizes of RNA standards are indicated to the left of the figure. A ³²P-labeled probe to 5S ribosomal RNA (arrow) was used to ensure loading uniformity. D, apPKG mRNA localization in pleural and pedal ganglia by in situ hybridization with a digoxigenin-labeled antisense RNA (left) or sense-RNA (right). apPKG mRNA is expressed in the pleural sensory cluster (arrow). Scale bar, 200 μm.

FIG. 2A-C. apPKG is a type-I PKG. A, The kinase activity of purified inactive recombinant apPKG protein (100 ng) was measured by the transfer of ³²P from [³²P]ATP to BPDEtide in the presence (+) or absence (−) of 100 nM 8-Br-cGMP. Activity caused by autophosphorylation was subtracted using a peptide control reaction (see Materials and Methods). Note that the kinase activity was inhibited in the presence of 10 μM PKG inhibitor Rp-8-pCPT-cGMPS (RP-G). B, Kinase activity of active recombinant apPKG (25 ng) and bovine PKG type-Iα (bPKG1α) (50 ng) in the presence of four type-I PKG peptide substrates: peptide A, RKISASGP (SEQ ID NO:21); B, RKISASEFDRPLR (SEQ ID NO:22; BPDEtide); and D, RKRSRAE (SEQ ID NO:23) H₂Btide). Peptide C, QKRPRRKDTP (SEQ ID NO:24), is a type-II PKG substrate. C, apPKG phosphorylates recombinant VASP at serine-239. Purified recombinant VASP-GST (0.5 μg) was incubated with active apPKG (100 ng) or the recombinant bovine PKG (50 ng), or in the kinase buffer alone, at room temperature for 20 min. After SDS-PAGE, a Western blot was probed with an anti-phospho-VASP (Ser 239) (p-VASP S239) antibody.

FIG. 3A-B. A, apPKG protein expression in the nervous system. Top, A Western blot (10 μg of protein per lane) was probed with antibody Ab apPKG raised against an N-terminal peptide of apPKG protein. Lane 1, Pedal ganglia; lane 2, pleural ganglia; lane 3, axoplasm extruded from peripheral nerves; lane 4, body wall muscle; lane 5, buccal mass; lane 6, genitalia; lane 7, recombinant apPKG. The two specific apPKG signals are indicated by arrows. Bottom, The blot was stripped and reprobed with an antibody against α-actin to indicate protein load. Positions of molecular mass markers in kilodaltons are indicated on the left. B, apPKG is expressed in the Aplysia SNs. Confocal microscopy of a 2 μm optical section taken from a Z-series through the pleural sensory cluster exposed to Ab apPKG 24 hr after nerve crush in vivo. Shown are representative images of contralateral uninjured (CC) and injured sensory clusters from sections in the middle of the neuron to show the nucleus. An injured sensory cluster stained in the presence of second Ab only shows the background staining. Scale bar, 20 μm. Although the staining is primarily in cytoplasm, the nucleus of some neurons is also stained. Note that the apPKG staining pattern is essentially identical between injured and contralateral uninjured sensory clusters.

FIG. 4A-C. ApPKG activity appears in the pleural ganglion after a delay after nerve crush. A, Peripheral nerves p5-p9 were crushed on one side. At the indicated times, pleural (white/gray squares) and pedal (dotted/black squares) ganglia were collected from the injured (black/gray) and contralateral control (CC) (white/dotted) sides and assayed for apPKG activity using BPDEtide as substrate in the presence of the PKA-specific inhibitor 6-22 amide. ApPKG activity at each point was corrected for autophosphorylation and was normalized to total apPKG activity elicited by adding 1_M 8-Br-cGMP to a duplicate sample. Relative apPKG activity was the ratio of the apPKG activity of each sample to the basal apPKG activity in a sample from a naive animal. Six animals were examined independently at each time point. Two-way ANOVA with repeated measures showed significant effects of axotomy, time, and their interaction in the pleural ganglia (p<0.001 in each case). B, apPKG is activated and retrogradely transported after injury. p5-p9 nerves were crushed and ligated. Twenty-four hours later, axoplasm was extruded from the crush (Cr) site, from the crush/ligation (Cr/Lig) site, and from the ligation (Lig site) on the control nerves, as indicated by brackets in the schematic. Axoplasm containing equal amounts of protein from each segment was assayed for apPKG activity as in A. The line indicates the level of basal apPKG activity, determined by assaying axoplasm collected from noninjured nerves. Error bars represent ±SEM. An asterisk indicates significant difference compared with all of the other groups (p<0.05; ANOVA and Newman-Keuls tests). An enrichment of active apPKG at the Cr/Lig site is characteristic of positive molecular injury signals. C, ApPKG protein is retrogradely transported after injury. Peripheral nerves were crushed and ligated as in B. Twenty-four hours later, injured and control nerves were fixed, exposed to Ab apPKG, and processed for immunohistochemistry. Optical sections (2 μm) through each nerve were examined by confocal microscopy. All of the images were at the same magnification and were captured after identical exposures to the confocal beam. Each image is aligned in the same direction; the central somata are to the left of the segment shown. 1, Segment of a nerve from a noninjured animal 2, Segment containing the Lig site (arrow) on a nerve contralateral to the injury. 3, Segment of nerve containing the Cr site (arrow), which has expanded over the 24 hr. 4, Segment of nerve containing the Cr/Lig site (arrow). 5, Segment of nerve exposed to the second antibody only. Scale bar, 20 μm for all of the images.

FIG. 5A-B. SNs develop an LTH in vitro. The electrical properties of SNs after time in vitro were compared with those in control SNs in the sensory cluster in vivo (d 0). A, Top, A representative single action potential elicited in response to a 20 msec depolarizing pulse showing the increase in spike amplitude after 7 d in vitro. Bottom, Action potential discharge in response to a normalized 1 sec intracellular test pulse. Note the repetitive firing in the neurons after 7 d in vitro. B, Data comparing spike duration, spike amplitude, spike threshold, and repetitive firing of control SNs (gray bars) with those after 2-7 d in vitro (open bars). Each bar contains the number of cells examined. Error bars represent ±SEM. An asterisk indicates significant difference from the in vivo value (p<0.01; ANOVA and Newman-Keuls test).

FIG. 6A-D. Inhibiting NOS, sGC, or apPKG prevents the induction of LTH in SNs in vitro. SNs were removed from the cluster and grown in vitro in the presence of Rp-8-pCPT-cGMPS (RpcGMPS), Rp-8-pCPT-cAMPS (RpcAMPS), or ODQ (all 10 μM). Other SNs removed at the same time were not exposed to any inhibitors as controls (C). A, B, On the third day in vitro, 12 SNs exposed to Rp-cGMPS, Rp-cAMPS, or ODQ, and 12 control SNs were impaled with a microelectrode to assess repetitive firing in response to stimulation at three test currents (A) and to determine spike threshold (B). We examined the cells on the third day, and not later, to avoid more prolonged exposure to the drugs. Two-way ANOVA with repeated measures showed that both Rp-cGMPS and ODQ significantly reduced repetitive firing elicited by test currents of 2 and 3 nA relative to controls. Error bars indicate SEM, and the asterisks indicate significance (p<0.0001 in each case). Similarly, Rp-cGMPS and ODQ significantly prevented the injury-induced decrease in threshold compared 4 with C cells (ANOVA and Fisher's PLSD tests; p<0.05). There was a considerable variability in threshold in the presence of the Rp-cAMPS, and the mean difference from controls was not significant. C, Detection of nNOS mRNA in single SNs by RT-PCR. Fragments of appropriate lengths were amplified with primer sets for apnNOS, sensorin A, and the neuron-specific isoform of actin from five separate sets of samples from SNs in vivo (0) or after 16 hr in vitro. The size of the synthesized fragments detected by ethidium bromide staining on 2% agarose gels were identical with those predicted from the known sequences in the database. In addition, the PCR products were verified by DNA sequence analysis. Finally, there was no amplification in the absence of reverse transcriptase, indicating the RNA preparations were not contaminated by genomic DNA (bottom panel). Positions of molecular markers are indicated on the left. D, Effects of the NOS inhibitors on LTH. L-Thiocitrulline (50 μM) and L-NAME (1 mM) were used as described above. n, Number of SNs. The data were normalized to the average excitability of control cells in the same preparation. The asterisk indicates significance (p<0.001), comparing LTH with and without inhibitor by ANOVA and Dunnett's test. Error bars indicate SEM.

FIG. 7A-F. A, Activation of apMAPK in pleural ganglia after nerve crush. Top, Left, Twenty-five micrograms of a pleural ganglia lysate collected at the indicated times after p5-p9 nerve crush were resolved by SDS-PAGE, and a Western blot was probed with Ab pTpYmapk to detect active ERK-MAPKs. The antibody recognized a 43 kDa kinase whose activity was increased on the injured side (I) relative to the contralateral control (CC) 16 hr and later after injury. Bottom, Left, The injury-activated kinase was apMAPK. The blot was stripped and probed with the D8 antibody, which recognizes both active and inactive apMAPK. D8 recognized the same 43 kDa protein that was recognized by the pTpY antibody. Right, Relative MAPK pTpY was determined by densitometry (Sung et al., 2003). All of the values were normalized to levels of total apMAPK. The ratio of the normalized MAPK pTpY intensity at each time to the normalized naive control is presented. The value for the naive control was arbitrarily set to 1.0. The Western analysis in this figure was performed with the same material used to assess apPKG activity in FIG. 4A. The apMAPK values represent an average of six animals. The line indicates the basal level of apMAPK activity from naive animals. Each of the following experiments (B-E) was repeated twice at least, and representative results are shown. B, apPKG phosphorylates endogenous apMAPK in neurons, but not axoplasm, in vitro. Left, Pleural neurons were removed from a noninjured animal, a lysate was prepared, and 25 μg was incubated with 100 ng of active apPKG protein or with 1 μM 8-Br-cGMP in the presence or absence of 10 μM U0126. Right, Twenty-five micrograms of axoplasm was incubated with active apPKG as above. Active apMAPK was detected by immunoblotting with Ab pTpYmapk. C, apPKG phosphorylates serine-383 in Elk1. One hundred nanograms of apPKG and 0.5 μg of purified recombinant Elk1 protein were incubated with either 5 μg of the pleural neuronal lysate or 0.2 μg of purified recombinant ERK2. Phosphorylated Elk1 (p-Elk1) was detected by probing a Western blot with an antibody that recognizes phosphorylated Ser 383. D, Direct phosphorylation of ERK2 at T183 by apPKG. Two hundred nanograms of recombinant ERK2 was incubated with 100 ng of apPKG in the presence or absence of 1 μg of BPDEtide. The reaction mixture was divided into thirds, a Western blot of each was prepared, and ERK2 was detected with Ab^(pTpYmapk) (top), Ab^(pYmapk) (middle), and Ab^(pTmapk) (bottom), respectively. E, Relative activation of ERK2 by apPKG and MEK1. One hundred nanograms of apPKG, the catalytic subunit of MEKK1, and MEK1 were used as indicated, and the production of phospho-Elk1 was measured with Ab^(pElk1) as shown in C. F, Nerve injury increases the level of apMAPK pY in the cell soma of injured (I) relative to contralateral control (CC) neurons. A Western blot prepared using 25 μg of a pleural neuronal lysate collected at the indicated times after p5-p9 nerve crush was probed with the monospecific pYantibody, Ab^(pYmapk), followed by D8 to detect total apMAPK as in A. The blot shows the results of two animals for each point. N, Lysate from animal without nerve crush.

FIG. 8A-B. A, apPKG does not enter the nucleus of SNs. Equal amounts of Alexa Fluor 546-labeled BSA, apPKG, or ERK1 were microinjected into the cytoplasm of SNs after 2 d in vitro. The neurites were then severed with a fine needle to elicit an injury response, and 30 min later, the cells were examined by confocal microscopy. The image is a 2 μm optical section through the center of the cell that shows the nucleus. Scale bar, 20 μm. B, Rp-8-pCPT-cGMPs inhibits axotomy-induced apMAPK nuclear translocation. SNs untreated as controls, and those exposed to either 10 μM Rp-8-pCPT-cGMPs (Rp-cGMPs) or Rp-cAMPs for 2 d in vitro, were immunostained with D8 antibody to localize apMAPK. Top, Representative examples of 2 μm optical sections of control SNs and those exposed to Rp-8-pCPT-cGMPs or Rp-cAMPs. Scale bar, 20 μm. Bottom, Histogram of the mean value of nuclear MAPK immunoreactivity. The staining intensity was determined by a person who was blind to the treatment that the cells received. n, Number of cells for each treatment. Asterisk indicates significant difference from control (p<0.05 by ANOVA and Newman-Keuls tests).

FIG. 9. Relative PKG activity at various positions of the sciatic nerve relative to point of injury, shown schematically above the graph.

FIG. 10A-C. A. balanol-7R; B. 10″ deoxybalanol; C. 14″ decarboxy balanol.

FIG. 11A-B. Surface dermatomes, A. front view, B. back view.

SEQUENCE LISTING

The specification further incorporates by reference the Substitute Sequence Listing submitted herewith via EFS on Mar. 11, 2008. Pursuant to 37 C.F.R. §1.52(e)(5), the Sequence Listing text file, identified as 070053091seqlist2.txt, is 77,465 bytes and was created on Feb. 25, 2008. The Substitute Sequence Listing, electronically filed herewith, does not extend beyond the scope of the specification and thus does not contain new matter.

5. DETAILED DESCRIPTION OF THE INVENTION

For clarity of description, and not by way of limitation, this section is divided into the following subsections:

-   -   (i) the NO/cGMP/PKG pathway;     -   (ii) assays for identifying modulators of the NO/cGMP/PKG         pathway; and     -   (iii) modulating the NO/cGMP/PKG pathway;     -   (iv) modulation of pain pathways by utilizing neuronal         retrograde transport mechanisms; and     -   (v) apPKG.

5.1 The NO/cGMP/PKG Pathway

The present invention provides a model for the induction of LTH and/or persistent pain in which nerve injury activates neuronal nitric oxide synthase (“nNOS’) in the axon, leading to NO production, resulting in the activation of soluble guanylyl cyclase (“GC”), the formation of cyclic guanosine monphosphate (“cGMP”), and the activation of protein kinase G (“PKG”). Activated PKG is retrogradely transported to the cell body of the sensory neurons (SNs) where it activates MAPK-erk in the cytoplasm, which then enters the nucleus and activates transcription of genes responsible for the appearance of LTH and/or persistent pain.

Long-term hyperexcitability (“LTH”), as defined herein, is increased sensitivity of a sensory neuron cell body or axon to stimuli. During electrophysiological testing, LTH is manifested as a decrease in the spike threshold, an increase in repetitive firing, broader spikes, and/or an increase in spike amplitude. In animals that perceive pain, LTH is associated with persistent pain (see Sung and Ambron, Mar. 22, 2004).

Electrophysiological testing may be performed using methods known in the art. One specific, non-limiting example of electrophysiological testing using Aplysia californica (hereafter referred to as either “Aplysia californica” or simply as “Aplysia”) SNs may be performed as follows (see Liao et al., 1999). Intracellular recordings from SN somata may be made with glass microelectrodes filled with 3 M potassium acetate (electrode resistance 8-20 M). Recordings may be made at 19-21° C. while the preparation is bathed in buffered artificial sea water (“ASW”), L15 medium, or a 1:1 mixture of ASW and L15, pH 7.6. Soma spike threshold may be measured with a standard series of 20 msec depolarizing pulses. Repetitive firing (spike accommodation) may be quantified by counting the number of spikes evoked by a 1 sec intracellular depolarizing pulse using 2.5 times the threshold current determined with the 20 msec pulse. Repetitive firing may, for example, be examined by counting the number of spikes evoked by a series of 1 sec depolarizing pulses at 1.25, 2.5, and 5 times the threshold current, or by 1, 2, 3, and 5 nA. Input resistance (Rin) may be determined from the voltage change produced during injection of a 1 sec hyperpolarizing pulse (0.5 nA). Axon excitability may be tested by passing current between two compartments through a narrow, Vaseline-sealed opening containing nerves p7, p8, and p9. Threshold may be determined with a rapid series of 2 msec pulses. Repetitive firing may be tested by applying two 1 sec pulses at 0.4 and 0.8 times the 2 msec threshold current.

Persistent pain includes pain that endures longer than the period of acute injury, and includes chronic pain syndromes such as, but not limited to, neuropathic pain (see Bennett et al., 2005). In specific, non-limiting embodiments, the duration of persistent pain is at least 1 day, at least one week, at least one month, or at least one year.

5.2 Assays for Identifying Modulators of the NO/cGMP/PKG Pathway

The present invention provides for assays that identify modulators (inhibitors or promoters/inducers) of the NO/cGMP/PKG pathway. Such assays may be used to evaluate a test agent in order to determine whether the test agent is an agent that modulates at least one step of the pathway and thereby modulates LTH. An inhibitor of the pathway may be used to inhibit LTH and may be used to inhibit and/or treat persistent pain in a sensory neuron and/or a subject. The term “inhibit,” as used herein, means lessen, delay, or prevent. A promoter/inducer of LTH may be used to develop a model system for persistent pain, preferably in an animal which, like Aplysia, is believed to not subjectively experience pain.

The assays of the invention utilize a model system which comprises a test sensory neuron (“TSN”) under physiological conditions which at least approximate the in vivo environment in which the sensory neuron exists in nature. The TSN comprises a cell body that contains the nucleus as well as an axonal segment, which constitutes at least a portion of the TSN's axon and more preferably constitutes the complete axon. In certain non-limiting embodiments, the TSN is an Aplysia SN. In other non-limiting embodiments, the TSN is a vertebrate SN, preferably a mammalian SN. The TSN may be maintained isolated in a culture, as part of a group of neurons which may or may not all be SNs, or as an explanted nerve or section thereof (e.g., an excised segment of rat sciatic nerve). In alternate embodiments, the TSN may be retained in an animal in vivo. In still further non-limiting embodiments, the axonal segment may contain at least one ligation.

The TSN is injured. For example, and not by way of limitation, the injury may be created by crushing, cutting and/or chemically injuring the TSN using methods known in the art. Other methods include inducing an inflammatory response, ischemia, a reduction of the blood supply to neurons, and hyperglycemia.

In the assays of the invention, a test agent is administered to the TSN, either prior to, concurrent with, or following injury, either comprised in culture medium, systemically administered, locally injected, or directly injected or otherwise introduced into the TSN. In non-limiting embodiments, the test agent may be administered to a particular cellular location of the TSN, such as the cell body or the axon. Preferably, the effects of the test agent on the TSN are compared to comparable values in a control SN (“CSN”), such as an injured CSN. Thereafter, one of the following evaluation steps is performed. In specific non-limiting embodiments, the evaluation steps are performed within 48 hours of injury.

In a first set of embodiments, the assay of the present invention determines whether the agent modulates nitric oxide synthase (“NOS”, which preferably is nNOS, but may also be eNOS and/or iNOS) activity in an injured TSN, preferably relative to NOS activity in an injured CSN to which test agent has not been administered. An ability to inhibit the increase in NOS (preferably nNOS) activity associated with SN injury indicates that the test agent is an LTH inhibitor. An ability to promote a further increase in NOS activity relative to control values indicates that the test agent is an LTH promoter. NOS activity may be measured, for example and not by way of limitation, by measuring either the amount of nNOS mRNA, the amount of nNOS protein, or the amount of nitric oxide (“NO”) produced. For example, nNOS mRNA may be measured by PCR amplification, using primers such as oligonucleotides having SEQ ID NOS; 15 and 16, below, or by in situ hybridization using a detectably labeled complementary oligonucleotide probe. For example, nNOS protein may be measured by immunohistochemistry using detectably labeled antibody (polyclonal or monoclonal) specific for nNOS. The amount of NO produced may be measured by, for example, and where the amount of cellular material is sufficient, measuring conversion of L-[¹⁴C] arginine to L-[¹⁴C]-citrulline as described in Smith et al., 2002.

In a second set of embodiments, the assay of the present invention determines whether the agent modulates guanylyl cyclase (“GC”) activity in an injured TSN, preferably relative to GC activity in an injured CSN to which test agent has not been administered. An ability to inhibit the increase in GC activity associated with SN injury indicates that the test agent is an LTH inhibitor. An ability to promote a further increase in GC activity relative to control values indicates that the test agent is an LTH promoter. GC activity may be measured, for example and not by way of limitation, by measuring either the amount of GC mRNA, the amount of GC protein, or the amount of cGMP produced. For example, GC mRNA may be measured by PCR amplification, using primers designed based on the GC nucleic acid sequence, and GC protein may be measured by immunohistochemistry using detectably labeled antibody (polyclonal or monoclonal) specific for GC. As one non-limiting example, where the volume of cellular material is appropriate, GC activity may be measured by a modification of the method set forth in Mo et al., 2004. For example, TSN may be lysed using a 2-fold concentrated lysis buffer containing 200 μM dithiothreitol and EDTA-free protease inhibitor, and the mixture may be sonicated and kept on ice. The lysed TSN may be assayed in 50 mM Tris-HCl buffer (pH 7.4) supplemented with 3 mM MgCl2, 0.1 mM EGTA, 0.05% (w/v) bovine serum albumin, 1 mM 3-isobutyl-1-methylxanthine, 5 mM creatine phosphate, 200 μg/ml creatine phosphokinase, 300 μM GTP, 1000 units/ml superoxide dismutase, 300 μM uric acid, and 200 μM cPTIO (2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). SPER/NO(N-4-1-3-aminopropyl-2-hydroxy-2-nitrosohydrazinobutyl-1,3-propane-diamine) was added, and aliquots may be removed at various intervals, inactivated, and assayed for cGMP. The amount of cGMP produced may be measured by, for example, and where the amount of cellular material is sufficient, by a chemiluminescence assay (HitHunter, Amersham Biosciences Corp., Piscataway, N.J.)

In a third set of embodiments, the assay of the present invention determines whether the agent modulates protein kinase G (“PKG”) activity in an injured TSN, preferably relative to PKG activity in an injured CSN to which test agent has not been administered. An ability to inhibit the increase in PKG activity associated with SN injury indicates that the test agent is an LTH inhibitor. An ability to promote a further increase in PKG activity relative to control values indicates that the test agent is an LTH promoter. PKG activity may be measured, for example and not by way of limitation, by measuring the kinase activity in a SN extract. For example, the amount of PKG activity in a SN extract may be determined by measuring the ability of extract to transfer ³²P from [³²P]-ATP to BPDEtide (Calbiochem, La Jolla, Calif.)

In a fourth set of embodiments, the assay of the present invention determines whether the agent modulates protein kinase G (“PKG”) transport in an injured TSN, preferably relative to PKG transport in an injured CSN to which test agent has not been administered. An ability to inhibit the transport of PKG associated with SN injury indicates that the test agent is an LTH inhibitor. An ability to promote a further increase in PKG transport relative to control values indicates that the test agent is an LTH promoter. PKG transport may be measured, for example and not by way of limitation, by determining whether an increase in PKG activity moves, over time, from the region of the SN that is injured toward the cell body. This increase may be measured by a number of methods, including, but not limited to, testing sequential, distal to proximal, axonal segments for PKG activity (see FIG. 9). In another non-limiting example, the axonal segment may be ligated and axonal material may be collected from regions of the axon at different relative distances from the injury and/or ligation, where, for example, the amount of activated PKG in the region of the injury becomes less, over time, relative to the amount of PKG activity on the side of the ligation facing the injury (see FIG. 4).

In a fifth set of embodiments, the assay of the present invention determines whether the agent modulates phosphorylation of mitogen-activated protein kinase-erk (“MAPKerk”) in an injured TSN, preferably relative to phosphorylation of MAPKerk in an injured CSN to which test agent has not been administered. An ability to inhibit phosphorylation of MAPKerk associated with SN injury indicates that the test agent is an LTH inhibitor. An ability to promote a further increase in phosphorylation of MAPKerk relative to control values indicates that the test agent is an LTH promoter. Phosphorylation of MAPKerk may be measured by determining the level of MAPKerk activity in phosphorylating its substrate, or by detecting the presence of phosphorylated MAPKerk using an antibody that selectively binds to phosphorylated rather than unphosphorylated protein (for example, but not by way of limitation Ab^(pTpYmapk).

In a sixth set of embodiments, the assay of the present invention determines whether the agent modulates MAPKerk translocation into the nucleus of an injured TSN, preferably relative to MAPKerk translocation into the nucleus of an injured CSN to which test agent has not been administered. An ability to inhibit MAPKerk translocation into the nucleus associated with SN injury indicates that the test agent is an LTH inhibitor. An ability to promote a further increase in MAPKerk translocation into the nucleus relative to control values indicates that the test agent is an LTH promoter. MAPKerk translocation into the nucleus may be measured by using a MAPKerk-specific antibody to measure the amount of MAPKerk in the SN nucleus using laser confocal immunohistochemical techniques.

5.3 Modulating the NO/cGMP/PKG Pathway

According to the present invention, the NO/cGMP/PKG pathway, and thereby the development of LTH and persistent pain, may be modulated using inhibitors disclosed in this section or promoters/inducers identified or having the properties set forth in the preceding section 5.2. In particular embodiments, an inhibitor may be administered to a sensory neuron in need of such treatment in an amount effective in inhibiting LTH. Where the SN to which the inhibitor is to be administered is a SN in vivo in an animal subject, the inhibitor may be administered systemically (e.g. by intravenous injection, oral administration, inhalation, etc.), may be injected locally (in proximity to the damaged nerve), may be applied topically (for example, together with a skin permeability enhancing agent, such as a chemical compound or an electrical stimulus) or may be administered by any other means known in the art, except that introduction into the central nervous system, such as intrathecal administration, would not be used to administer inhibitor to a SN. The amount of inhibitor to be administered may be determined using methods known in the art, for example, by doing dose response studies in one or more model system, such as the Aplysia system described above or a mammalian model of peripheral neuropathic pain, followed by approved clinical testing in humans. Where concentrations are set forth below, they refer to the concentration to which the sensory neuron or any component thereof, including axon, cell body or receptor, is exposed.

In related embodiments, an effective amount of an inhibitor may be administered to a subject in need of such treatment, where the subject suffers from chronic pain. The chronic pain preferably has a peripheral nervous system (primary) hyperalgesia component, where the method inhibits pain mediated by the peripheral nervous system, but in specific non-limiting embodiments the present invention also encompasses the treatment of spinal hyperalgesia as either a component of or the basis of (e.g., chronic central neuropathic pain resulting from spinal cord injury) chronic pain. Any of the foregoing modes of administration may be used, but if a spinal hyperalgesia component is to be treated, the inhibitor, which is directed to a neuron having its cell body in the central nervous system and not in the dorsal root ganglion, should be administered intrathecally.

An effective amount is an amount of inhibitor which decreases the level of pain subjectively perceived by the subject, preferably amount determined, in controlled experiments, which is greater than placebo effect. For example, and not by way of limitation, in certain embodiments of the invention, where perceived pain can be quantified on a scale from 0 to 10, where 0 is no pain, 1-5 is progressively more intense mild pain, 6-7 is progressively more intense moderate pain, 8-9 is progressively more intense severe pain, and 10 is the worst pain possible, an effective amount of inhibitor may decrease the pain scale quantification of perceived pain by at least 2 points, or by at least 3 points.

In specific, non-limiting embodiments, the present invention provides for a method for treating chronic pain in a subject comprising administering, to the location from which the pain arises, an effective amount of an inhibitor as set forth herein (alternatively referred to as an “LTH inhibitor”), where administration can be by local injection or topical application (e.g., via a cream, ointment, or transdermal device, which may be a patch or may be an apparatus or an apparatus containing or otherwise associated with a patch), and the location can be, as non-limiting examples, a wound site, tissue overlying an inflamed joint, or an area within the dermatome associated with the perceived pain (e.g., L4, L5, S1, C3, C4, C5, C6 or C7, see below).

5.3.1 Modulation of NOS

In one specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an inhibitor of NOS (preferably nNOS, and/or eNOS and/or iNOS), such as, but not limited to, L-NAME [N^(G)-nitro-L-arginine methyl ester hydrochloride], L-thiocitrulline, or an antisense nucleic acid or RNAi that inhibits NOS expression. Such an antisense nucleic acid molecule or RNAi may hybridize to the target NOS under stringent hybridization conditions, as defined below.

5.3.2 Modulation of NO Levels

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent which decreases NO levels, including, but not limited to, quercetin (which is an NO scavenger (Griffiths et al., 2003)).

5.3.3 Modulation of GC

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent that inhibits the increase in GC activity, such as, but not limited to, L-H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (“ODQ”), or an antisense nucleic acid molecule or RNAi that inhibits expression of GC. Such an antisense nucleic acid molecule or RNAi may hybridize to the target GC under stringent hybridization conditions, as defined below.

5.3.4 Modulation of cGMP Levels

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent that decreases cGMP levels, such as, but not limited to, an agent that increases phosphodiesterase, preferably PDE5, activity, such as, but not limited to, administering phosphodiesterase (e.g. PDE5) itself, or the PDE5 activator Ant-cGMP-2′-O-anthranyloyl cGMP (Guanosine 3′,5′ cyclic monophosphate, 2′-O-anthraniloyl cGMP).

5.3.5 Modulation of PKG Activity

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent that inhibits PKG activity. In specific, non-limiting embodiments, the agent inhibits activated PKG and/or its arrival from the axon in a neuronal cell body.

In non-limiting embodiments, the agent is a peptide inhibitor of PKG. Non-limiting examples of peptide inhibitors of PKG are disclosed in Dostmann, 2000, and include a peptide comprising the sequence LRKKKKKH (SEQ ID NO:26), LRAKKKKH (SEQ ID NO:27), LRKAKKKH (SEQ ID NO:28), LRKKAKKH (SEQ ID NO:29), LRKKKAKH (SEQ ID NO:30) or LRKKKKKH (SEQ ID NO:31). In other embodiments, a peptide inhibitor of PKG may comprise a core sequence, RKK or RKKK (SEQ ID NO:32), and may be between about 5 and 100, or between 5 and 50, or between 10 and 30, or between 10 and 20, amino acids long.

A peptide inhibitor of PKG according to the invention may further comprise one or more transport peptide, one or more carrier peptide, or both transport and carrier peptides, as well as additional peptide or non-peptide components which may improve transport, effectiveness, and/or stability. Thus, the present invention provides for peptides comprising inhibitor-Δ-carrier, carrier-Δ-inhibitor, inhibitor-Σ-transport, transport-Σ-inhibitor, carrier-Δ-inhibitor-Σ-transport, and transport-Σ-inhibitor-Δ-carrier, where Δ and Σ are optional linker peptides, carrier is a carrier peptide, inhibitor is an inhibitor peptide, and transport is a transport peptide, as set forth herein, where in non-limiting embodiments the size of the entire peptide is between 10 and 100, or between 10 and 50, or between 10 and 30 amino acids long. In specific, non-limiting embodiments of the invention, the present invention provides for peptides comprising LRKKKKKHΔYGRKKRRQRRRPP (SEQ ID NO:33), YGRKKRRQRRRPPΔLRKKKKKH (SEQ ID NO:34), LRKKKKKHΔRQIKIWFQNRRMKWKK (SEQ ID NO:35), RQIKIWFQNRRMKWKKΔLRKKKKKH (SEQ ID NO:36), LRKKKKKHΣPKKKRK (SEQ ID NO:37), PKKKRKΣLRKKKKKH (SEQ ID NO:38), YGRKKRRQRRRPPΔLRKKKKKHΣPKKKRK (SEQ ID NO:39), PKKKRKΣLRKKKKKHΔYGRKKRRQRRRPP (SEQ ID NO:40), RQIKIWFQNRRMKWKKΔLRKKKKKHΣPKKKRK (SEQ ID NO:41), PKKKRKΣLRKKKKKHΔRQIKIWFQNRRMKWKK (SEQ ID NO:42), where Δ and Σ are optional linker molecules of between 0 and 5, or between 0 and 10, or between 0 and 20, amino acids, which peptides may be between 5 and 100, or between 5 and 50, or between 10 and 30 amino acids long. The present invention further provides for peptide inhibitors which are at least about 90 or about 95 percent homologous to the above-recited peptides, as determined using standard homology assessing software such as BLAST or FASTA, and which inhibit PKG, and which may be between about 5 and 100, or between 5 and 50, or between 10 and 30, or between 10 and 20, amino acids long. In specific, non-limiting embodiments, the effective concentration of peptide inhibitor may be between 1 nanomolar and 10 micromolar.

In other non-limiting embodiments, the inhibitor of PKG is a compound of formula I, as set forth below:

In Formula I, n is 1, 2 or 3; Z is N or CH In Formula I, X represents one of the following functional groups:

In Formula I, Y represents one of the following functional groups:

In Formula I, A represents aryl or heteroaryl groups un-substituted or substituted by one or more lower-alkyl, lower-alkoxy, hydroxy, alkoxy, amino, alkylamino or halogen groups. Examples of the aryl or heteroaryl groups are listed below:

-   -   When Y is:

-   -   A is:

-   -   When Y is:

-   -   A is:

In Formula I, R is hydrogen, lower-alkyl, or amidino. In Formula I, R1, R2, R4, R5 is independently hydrogen, hydroxyl, lower-alkoxy, amino, halogen. In Formula I, R3 is alkyl, aryl, heteroaryl, alkoxy, aryloxy, or one of the groups listed below:

where R6-R10 are independently hydrogen, hydroxy, lower-alkyl, lower-alkoxy, halogen, trifluoromethyl, carboxy, alkoxycarbonyl, amino, alkylamino, alkylcarbonylamino, alkylsulfonylamino (e.g. CF₃SO₂NH—, CH₃SO₂NH—), tetrazole. In one specific, non-limiting embodiment, the compound of formula I is balanol or a derivative thereof, as disclosed in Internation Patent Application No. PCT/US92/07124, Publication No. WO93/03730, where balanol has formula Ia, as follows:

In particular nonlimiting embodiments of the invention, Formula Ia may be varied to provide “balanol variants” which inhibit PKG. Non-limiting examples of such balanol variants include balanol-7R, 14″ decarboxy balanol, and 10″ deoxybalanol, as set forth in FIG. 10A-C, which are inhibitory of PKG at concentration of 291, 19 and 31 nM, respectively, where balanol is inhibitory at 1.6 nM (see Setyawan et al., 1999).

The present invention further provides for molecules of Formula I, Formula Ia, and balanol variants which are conjugated to one or more carrier peptide, one or more transport peptide, or one or more carrier peptide and one or more transport peptide (also referred to as balanol variants, or balanol double variants). In specific non-limiting embodiments, the concentration of balanol or balanol variant administered to the neuron, for example via its axon, may be between about 1 and 500 nM, or between about 2 and 100 nM, depending on the potency of the compound.

In another specific, non-limiting embodiment, the agent is Rp-8-pCPT-cGMPS. In a related embodiment, the agent is Rp-8-pCPT-cGMPS conjugated to one or more transport peptide. In another related embodiment, the agent is Rp-8-pCPT-cGMPS conjugated to one or more carrier peptide. In yet another related embodiment, the agent is Rp-8-pCPT-cGMPS conjugated to both one or more transport peptide and one or more carrier peptide. (see below for definition of transport and carrier peptides). In specific, non-limiting embodiments, the concentration of Rp-8-pCPT-cGMPs may be between 1 micromolar and 500 micromolar.

In another specific, non-limiting embodiment, the agent is an antisense nucleic acid molecule or RNAi that inhibits expression of PKG. Such an antisense nucleic acid molecule or RNAi may hybridize to the target PKG under stringent hybridization conditions, as defined below.

5.3.6 Modulation of PKG Transport

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent that inhibits PKG transport.

In non-limiting embodiments of the invention, an agent that inhibits PKG transport may comprise a transport peptide, as set forth below, in an amount effective to inhibit the axonal retrograde transport system. In non-limiting specific embodiments, an agent that inhibits PKG transport may comprise a plurality of transport peptides, for example, but not by way of limitation, comprised in surface loops of a decoy molecule.

5.3.7 Modulation of MAPKerk Activity

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent that inhibits activation (phosphorylation) of MAPKerk. Such molecules include, but are not limited to, tyrosine kinase inhibitors such as K252a and genistein.

5.3.8 Modulation of MAPKerk Transport

In another specific non-limiting embodiment, the present invention provides for inhibiting the NO/cGMP/LTH pathway, LTH, and/or persistent pain, by a method comprising administering, to a SN (which may or may not be a SN in vivo in a subject), an effective amount of an agent that inhibits translocation of MAPKerk into the nucleus, including, but not limited to, a calcium antagonist such as felodipine (Yang et al., 2002), verapamil, diltiazem, nifedipine, etc., or apolipoprotein D (Sarjeant et al., 2003).

5.3.9 Compositions for Treating Pain

Compositions of the invention may comprise an inhibitor agent as described above, where the inhibitor agent optionally comprises a carrier molecule which facilitates its translocation through a neuronal cell or nuclear membrane. Examples of carrier molecules which may be used include but are not limited to HIV-1 tat protein (YGRKKRRQRRRPP; SEQ ID NO:43) and peptides that are about 9-30 or about 9-20 residues long comprising its cores sequence RKKRRQRRR (SEQ ID NO:44), Drosophila Antennapedia homeo-domain (RQIKIWFQNRRMKWKK; SEQ ID NO:45). Other carrier molecules that may be used according to the invention may be largely comprised (contain at least 60 percent, at least 70 percent, or at least 80 percent) of positively charged amino acids such as arginine (Wender et al., 2000) and/or lysine (Mai et al., 2002). Also encompassed by the invention are peptides and derivatized peptides which are at least about 90 or about 95 percent homologous to the above-recited peptides, as determined using standard homology assessing software such as BLAST or FASTA. The inhibitor agent may optionally alternatively or additionally comprise a transport peptide, as described below.

The present invention provides for such inhibitor agents, in either lyophilized form or dissolved in a suitable pharmaceutical carrier. Compositions that comprise more than one inhibitor agent are encompassed by the invention.

In non-limiting embodiments, the invention provides for a pharmaceutical composition comprising one or more inhibitor agent, as set forth above, together with at least one agent that promotes uptake of the inhibitor agent into a peripheral nerve. Examples of such agents include membrane permeability enhancing agents such as dimethyl sulfoxide and/or 2 hydroxypropyl-b-cyclodextrin.

In other non-limiting embodiments, the invention provides for a pharmaceutical composition comprising one or more inhibitor agent, as set forth above, together with at least one agent that treats an underlying cause of the pain, including, but not limited to, an anti-inflammatory agent (such as aspirin, a non-steroidal anti-inflammatory agent such as ibuprofen, or a corticosteroid).

In other non-limiting embodiments, the invention provides for a pharmaceutical composition comprising one or more inhibitor agent, as set forth above, together with at least one agent having a local anesthetic effect, such as lidocaine.

In a further non-limiting embodiment, the present invention provides for a transdermal device, such as a patch or apparatus comprising one or more inhibitor agent, as set forth above, and optionally one or more additional agent which promotes the uptake of agent in a peripheral nerve, treats an underlying cause of the pain, and/or has local anesthetic effect, where exemplary compounds in each of these categories is provided above. The device may in general utilize transdermal patch technology known in the art, to facilitate sustained release of its therapeutic agents through the skin of a subject. In specific, non-limiting embodiments, the device creates an electrical potential which promotes uptake of the inhibitor agent(s) into local tissue (iontophoresis) or improves drug transfer using ultrasound or radiofrequency waves (see Bryan, 2004; U.S. Pat. No. 5,405,614, U.S. Pat. No. 4,708,716).

5.4 Modulation of Pain Pathways by Utilizing Neuronal Retrograde Transport Mechanisms

The present invention provides for a method for modulating and specifically inhibiting pain pathways, the perception of pain and primary (peripheral nervous system) hyperalgesia, comprising delivering a pain inhibitor compound to an axon of a sensory nerve such that the pain inhibitor compound is retrogradely transported along the axon to the sensory neuron cell body in the dorsal root ganglion. According to this embodiment of the invention, the pain inhibitor compound is not limited to the LTH inhibitors set forth herein, but may be any pain inhibitor that operates on sensory neurons, such as prostaglandin inhibitors (e.g., COX-2 inhibitors), peripherally acting opioids, anesthetic compounds, etc., linked to a transport peptide which facilitates retrograde axonal transport.

In one non-limiting example, the transport peptide is PKKKRK (SEQ ID NO:46), or a peptide or derivatized peptide which is at least about 80 percent homologous thereto as determined using standard homology assessing software such as BLAST or FASTA and which facilitate axonal transport. In another non-limiting example, the transport peptide is the related peptide CTPPKKKRKV (SEQ ID NO:47)(see Ambron, 1992), or a peptide or derivatized peptide which is at least about 70, at least about 80, or at least about 90 percent homologous thereto as determined using standard homology assessing software such as BLAST or FASTA and which facilitate axonal transport. In specific, non-limiting embodiments of the invention, the transport peptide is between 5 and 20 amino acids long and comprises the peptide KKKRK (SEQ ID NO:48), PKKKRK (SEQ ID NO:46), PPKKKRK (SEQ ID NO:49), TPPKKKRK (SEQ ID NO:50), or PKKKKRKV (SEQ ID NO:51).

For example, the pain inhibitor compound (comprising an agent that inhibits pain associated with a sensory neuron and a transport peptide; preferably an LTH inhibitor as set forth above) may be delivered to a peripheral pain receptor at the site of injury or in the same dermatome as the injury, as sensory axons arising throughout the dermatome converge on the same dorsal root ganglion. FIG. 11A-B presents the sensory dermatomes (from The Merck Manual of Diagnosis and Therapy, Section 14, Chapter 165, FIG. 165-2, which references Keegan J J and Garrett F D, “Anatomical Record 102:409-437, 1948, used with permission of the Wistar Institute, Philadelphia, Pa.). As examples, arthritis pain associated with the fingers is communicated via axons whose cell bodies reside in DRGs at levels C5-T1 and pain from the knees is communicated via axons whose cell bodies reside in DRGs at levels L3-S2.

Accordingly, the present invention provides for a method of treating pain in a subject, where the pain is determined to be associated with a dorsal root ganglion at a particular spinal cord level, comprising topically applying a pain inhibitor comprising a transport peptide to skin lying within the dermatome corresponding to the spinal cord level associated with the pain.

The pain inhibitor compound may be comprised in a cream, ointment, or transdermal device (see above), applied to the appropriate dermatome.

For example, a person suffering from lower back pain as a result of compression of the nerve exiting a bony foramen in the lower spine (lumbar, sacral, or lumbosacral radiculopathy) could be treated with a transdermal patch containing a pain inhibitor compound (comprising a transport peptide) applied to the dermatome corresponding to the spinal cord level from which the compressed nerve originates, which may be identified by the person's symptoms and physical exam. As one specific example, because the radiculopathy often involves nerves that supply the L4, L5 and/or S1 dermatomes, a transdermal patch according to the invention may be applied to the buttock of the patient. As another specific non-limiting example, a person having arthritis involving the finger joints, dermatomes C6-C8, could wear a patch according to the invention on the upper arm or shoulder, for example above the spine of the scapula.

This aspect of the invention provides the advantage that it treats pain while avoiding systemic administration of antinociceptive compound, thereby decreasing substantially the potential for substance abuse and avoiding potential side effects, such as those associated with COX-2 inhibitors.

5.5 apPKG

The present invention further provides for nucleic acid molecules encoding protein kinase G as well as their encoded proteins.

In certain non-limiting embodiments, the present invention provides for an isolated nucleic acid that encodes a protein kinase G of Aplysia having the amino acid sequence set forth in GenBank Acc. No. AY362340 and SEQ ID NO:10 herein. In particular embodiments, the present invention provides for an isolated nucleic acid molecule having a sequence as set forth in GenBank Acc. No. AY362340 and SEQ ID NO:9 herein, as well as nucleic acid molecules that are at least 85, at least 90, or at least 95 percent homologous thereto, where homology is determined by standard homology determining software such as BLAST or FASTA. The present invention further provides for isolated nucleic acid molecules that hybridize to a nucleic acid molecule having SEQ ID NO:9 under stringent conditions, and that encode a molecule having protein kinase activity. Stringent conditions are defined herein as hybridization to filter-bound DNA in 0.5 M NaHPO4, 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at 65° C., and washing in 0.1×SSC/0.1% SDS at 68° C. (Ausubel F. M. et al., eds., 1989, Current Protocols in Molecular Biology, Vol. I, Green Publishing Associates, Inc., and John Wiley & sons, Inc., New York, at p. 2.10.3).

The nucleic acids of the invention may be comprised in a vector molecule, which may be a phage, plasmid, phagemid, or virus.

The nucleic acids of the invention may be operably linked to a promoter element to create an expression cassette.

The present invention further provides for an isolated protein having a sequence as set forth in SEQ ID NO:10, as well as proteins that are at least 85, at least 90, or at least 95 percent homologous thereto and exhibit protein kinase activity. The proteins of the invention may be comprised in fusion proteins. As one non-limiting example, the proteins of the invention may be fused to an immunologically recognizable tag, such as a His tag (see Section 6, below).

In further embodiments, the present invention provides for an isolated protein comprising the catalytic portion of apPKG, including a catalytic portion of apPKG having the amino acid sequence:

VAKEFENCSLDDLQLVTTLGMGGFGRVELVQLSKEK (SEQ ID NO:25) GKTFALKCLKKKHIVETRQQEHIYSEKKIMMEADSP FITKLHKTFRDRKYVYMLMEVCLGGELWTILRDRGN FDDLTARFCVACVLEAFSYLHAKGIIYRDLKPENLL LDARGYVKLVDFGFAKKIGVGKKTWTFCGTPEYVAP EIILNKGHDHSADYWSLGILMYELLNGTPPFSGSDP MRTYNIILKGIDHIEFPKKISRSAHVLIKKLCRDNP MERLGYGKNGISDIRKNKWF, and amino acid sequences that are at least 85 percent, at least 90 percent, or at least 95 percent homologous thereto. The catalytic portion of apPKG, and molecules that are at least 85, at least 90, or at least 95 percent homologous thereto, may be comprised in a fusion protein, linked to another amino acid sequence at the N and/or C terminal end. The present invention further provides for isolated nucleic acid molecules encoding said isolated proteins, which may be operably linked to a promoter element comprised in an expression cassette and/or a vector molecule.

The present invention further provides for antibody molecules that are prepared by immunizing an animal with a purified protein according to the invention. Such antibody molecules may be polyclonal or monoclonal, prepared using standard laboratory techniques.

6. EXAMPLE A Neuronal Isoform of Protein Kinase G Couples Mitogen-Activated Protein Kinase Nuclear Import to Axotomy-Induced Long Term Hyperexcitability in Aplysia Sensory Neurons 6.1 Materials and Methods

In vivo nerve crush. Aplysia (100-150 gm) were anesthetized with isotonic MgCl2, and a small incision was made on one side of the body wall. Pedal nerves 5-9 were crushed 2 cm from the pedal-pleural ganglia on one side. The wound was sutured, and the animal was returned to its tank. The crush-ligation protocol followed was as described (Ambron et al., 1995).

Cloning. Degenerate oligonucleotide primers 5′-tayaaytgyacnmgiacngc (SEQ ID NO:1) and 5′-ccrcaraangtccangtytt (SEQ ID NO:2) were used to amplify an apPKG cDNA fragment from Aplysia CNS cDNA. The resulting PCR product from this amplification was cloned into pCR-II (Invitrogen, Carlsbad, Calif.) and subsequently sequenced by the core facility at Columbia University. The 5′ end and 3′ end of the cDNA were cloned using 5′-rapid amplification of cDNA ends (RACE) and 3′-RACE, respectively. A Marathon cDNA Amplification kit (BD Clontech, Palo Alto, Calif.) was used to generate cDNA from Aplysia CNS poly(A+) RNA according to the manufacturer's instructions. For the 5′-RACE reaction, a specific primer, 5′-cgcctgtccagcacccatagcg (SEQ ID NO:3), was used. The product of this PCR reaction was then confirmed by a second amplification using a nested, specific 5′ primer, 5′-gggtgaccgctttcacggagg (SEQ ID NO:4). For the 3′-RACE reaction, a specific primer, 5′-cggcaaggttctgcgtcgcc (SEQ ID NO:5), was used. The PCR product was then subjected to a second amplification using a nested, 3′ primer, 5′-ggacgcgaggggatacgtc (SEQ ID NO:6). Both 5′- and 3′-RACE products were subcloned into pCR-II and sequenced. To obtain the full-length cDNA in one piece, another PCR was performed with oligonucleotides 5′-ggtggaggagatagcggcggttctgtgaacgcc (SEQ ID NO: 7) and 5′-ggaggagtgagggtcagatcc (SEQ ID NO:8), corresponding to the 5′ and 3′ ends of 5′- and 3′-RACE products, respectively. The PCR product was sequenced and designated apPKG and was deposited in the GenBank database under accession number AY362340 (SEQ ID NO:9) The deduced amino acid sequence is SEQ ID NO:10.

Sequence analysis. Sequence alignment of various PKGs and identification of conserved residues was performed using the Clustal W and box-shade algorithms provided in the suite of programs available from Biology Workbench. Protein expression and purification. A His tag was added to the N terminus of the apPKG-coding region by PCR amplification from Aplysia CNS cDNA with the following primers: 5′-tggcggccgctcatgagaggatcgcatcaccatcaccatcacggcaacggtgccagttcgaacacgcacttc (SEQ ID NO:11) and 5′-gcaggctctagagaaatcaatgtcccagccggataactcgtc (SEQ ID NO:12). The PCR product was subcloned into the NotI and XbaI sites of pFasBac-1 (Invitrogen) and was subsequently confirmed by sequencing. The resulting construct pFB1apPKG contains an N-terminal histidine epitope tag. Transformation of pFB1apPKG into Max Efficiency DH10Bac cells (Invitrogen), identification of recombinant clones, and isolation of the recombinant baculovirus shuttle vector DNA (bacmid) were performed according to the manufacturer's instructions (Invitrogen). Recombinant baculovirus was obtained by transfecting Sf9 cells (Spodoptera frugiperda), which were propagated as monolayers at 27° C. in Sf-900 IISFM medium (Invitrogen) containing 100 U/ml penicillin (Invitrogen) and 0.1 mg/ml streptomycin (Invitrogen). Transfection with recombinant bacmid DNA was performed using CellFectin (Invitrogen) according to the instructions of the manufacturer. Positive viral clones were identified by their ability to direct the expression of the appropriate protein as revealed by immunoblotting of whole-cell extracts of transfected Sf9 cells harvested 3 d posttransfection using an antibody to the His tag of the protein. For apPKG proteinexpression, Sf9 cells were infected with the recombinant baculovirus at a multiplicity of infection of greater than 10. After 72 hr, cells were harvested and recombinant His-apPKG was purified on nickel nitriloacetic acid resin (Qiagen, Valencia, Calif.) according to the manufacturer's instructions. To express VASP (vasodilator-stimulated phosphoprotein), the coding region of VASP was first obtained by PCR amplification from mouse brain cDNA with the following primers: 5′-gtcgtgggatccccatcgatagcgagacggtcatctgt (SEQ ID NO:13) and 5′-atcttgaattcctcgagggtcaaggagaaccccgctt (SEQ ID NO:14). The PCR product was subcloned into the EcoRI and BamHI sites of pGEX3X (Amersham Biosciences, Arlington Heights, Ill.) and was subsequently confirmed by sequencing. The resulting construct pGEXVASP contains a C-terminal GST (glutathione S-transferase) epitope tag. VASP-GST, Elk1-GST, MEKK1C (MAP kinase kinase kinase 1C)-GST, MEK1 (MAP kinase kinase 1)-GST, and ERK1-GST fusion proteins were expressed in Escherichia coli DH5α and purified as described (Sung et al., 1996).

Northern blots. Total RNA was extracted from various tissues and resolved by denaturing agarose gel electrophoresis; the gel was then transferred to a nylon membrane. The resulting blot was hybridized with radioactively labeled apPKG and 5S ribosomal cDNA as described previously (Alberini et al., 1994; Sung et al., 2001).

Single-cell RT-PCR. Single SNs were transferred to 500 μl of Tri Reagent (Molecular Research Center, Cincinnati, Ohio), and total RNA was isolated according to the manufacturer's instructions. cDNA from each sample was synthesized using random hexamers as primers and reverse transcriptase (SuperScript II). Aliquots (2 μl) from each sample were used to amplify specific fragments by PCR (40 cycles), using specific primer sets for the following: (1) neuronal nitric oxide synthase (NOS) (GenBank accession number AAK83069), 5′-gtaccctcacaggacgagtc (SEQ ID NO:15) and 5′-tccttggacctctcttggtg (SEQ ID NO:16) (nt 3610-4049); (2) SN-specific neuropeptide sensorin A (GenBank accession number X56770), 5′-aacagaaacagtctttcccc (SEQ ID NO:17) and 5′-tcttgactcaccaactgcc (SEQ ID NO:18) (nt 43-331); and (3) neuron-specific actin (GenBank accession number U01352), 5′-cagagagaagatgacccag (SEQ ID NO:19) and 5′-gggtaagagaagcaagaaag (SEQ ID NO:20) (nt416-1298).

Kinase assays. In vitro PKG activity was measured as described (Pohler et al., 1995). Briefly, 100 ng of His-apPKG was incubated with 5 μg of various peptides in a buffer containing the following (in mM): 25 Tris-HCl, pH 7.5, 5 β-glycerol phosphate, 2 DTT, 0.1 Na₃VO₄, and 10 MgCl₂. The reaction was initiated by adding 10 μM [γ-³²P]ATP. The incubation was allowed to proceed for 20 min at room temperature and terminated with 50 mM EDTA (final concentration). Labeled peptides were captured on P81 filters (Whatman, Maidstone, UK). The filters were washed with 0.5% phosphoric acid and dried, and the bound ³²P-phosphopeptide was detected by liquid scintillation counting. All of the values were corrected for background counts per minute obtained without the substrate. To evaluate endogenous apPKG activity, 5 μg of ganglia extract or axoplasm was used in the kinase buffer (above) with 5 μg of PKA inhibitor and in the presence or absence of 1 μM cGMP. ERK activity was assayed as described (Sung et al., 2001). Briefly, proteins and 500 μM ATP were incubated in kinase buffer for 20 min at room temperature. The reaction mixtures were resolved on 10% SDS polyacrylamide gels and subjected to Western blotting with antibody (Ab)pTpYmapk, AbpYmapk, AbpTmapk, or AbpElk1.

In situ hybridization. Ganglia were first isolated from animals, desheathed, and fixed in 4% paraformaldehyde in PBS, pH 7.4, for 3 hr. The ganglia were then washed several times in 1×PBS and then digested with 80 μg/ml proteinase K (Ambion, Austin, Tex.) in 1 μPBS for 30 min at room temperature (RT). After several washes in 1×PBS, the ganglia were fixed again for 20 min with 4% paraformaldehyde and then washed several more times in 1×PBS. After treatment with 1.32% triethanolamine HCl, pH 8.0 (10 min at RT), and 0.24% acetic anhydride in 1.32% triethanolamine HCl, pH 8.0 (20 min at RT), and several washes with 1×PBS, the ganglia were prehybridized in Hyb buffer (50% formamide, 5×SSC, 5×Denhardt's solution, 0.25 mg/ml yeast tRNA, and 0.5 mg/ml salmon sperm DNA) at 60° C. for 2 hr, and then hybridized overnight at 60° C. with fresh Hyb buffer containing either antisense or sense digoxigenin (DIG)-labeled cRNA (1 μg/ml). After hybridization, ganglia were first washed for 30 min in fresh Hyb buffer at 68° C., and then in 0.2×SSC at 68° C. for 1 hr. After equilibration in PBST (1×PBS and 0.1% Triton X-100), ganglia were blocked with 10% sheep serum in PBST for 30 min at RT, and then incubated with anti-DIG antibody (1:5000) coupled to alkaline phosphatase (Roche, Indianapolis, Ind.) in 1×PBST containing 1% sheep serum overnight at 4° C. Hybridization signals were visualized with nitroblue tetrazolium chloride/5-bromo-4-chloro-3-indolyl phosphate (Roche).

Western blotting. Protein samples were resolved on 10% SDS polyacrylamide gels and subsequently transferred onto nitrocellulose membranes (Schleicher & Schuell, Keene, N.H.); the blots were probed with various gene-specific primary antibodies and appropriate horseradish peroxidase-conjugated secondary antibodies. Immunoreactivity was detected using the Pico-tag chemiluminescence system (Pierce, Rockford, Ill.).

Immunocytochemistry. Ganglia, nerve, or cultured cells were fixed with 4% paraformaldehyde in PBS, pH 7.4. The primary antibody was diluted in TBS supplemented with 0.1-0.5% Triton X-100 in TBS and 5% goat serum, and incubated overnight at 4° C. After several washes, an Alexa Fluor 594- or 488-conjugated secondary antibody (Molecular Probes, Eugene, Oreg.) was applied for 1 hr at room temperature. Subsequently, the cells were visualized by confocal fluorescence microscopy (LSM510 confocal microscope; Zeiss, Oberkochen, Germany), and images were collected.

Cell culture. SNs were isolated from the pleural ganglia of 50-80 gm animals and were plated on poly-L-lysine-coated dishes containing L15 medium and 50% hemolymph (Dagan and Levitan, 1981; Glanzman et al., 1989). Cultures were maintained at 18° C. for up to 7 d. Medium was changed every 2 d. Drugs were washed out 1 hr before electrophysiological tests.

Electrophysiology. Before the start of each recording, the hemolymph was replaced with a 1:1 mixture of artificial seawater and culture medium (without hemolymph; pH 7.6). Standard techniques were used for intracellular stimulation and recording (Ambron et al., 1996). The soma spike threshold was measured with a standard series of 20 msec depolarizing pulses. Repetitive firing was quantified by counting the number of spikes evoked by a series of 2 sec depolarizing pulses at 1, 2, 3, 4, and 5 nA, or 1 sec depolarizing pulses at 2.5 times the current for the 20 msec threshold. Spike amplitude was measured from baseline to the peak of the action potential, and spike duration was the breadth of the action potential at one-half of its maximal height.

Fluorescence protein labeling. BSA, apPKG, and ERK1 protein were labeled using an Alexa Fluor 546 Protein Labeling kit (Molecular Probes) according to the manufacturer's instructions.

SN microinjection. Microinjection pipettes were prepared with a Sutter programmable puller. Alexa Fluor 546-labeled protein (0.75 μg/μl in 10 mM Tris-HCl, pH 7.3, 100 mM NaCl, and 0.05% fast green dye) was microinjected into cultured SNs by applying positive air pressure under defined conditions (pounds per square inch and duration) using a picospritzer (Sung et al., 2000).

Materials. Recombinant bovine PKG 1α, guanosine 3′,5′-cyclic monophosphorothioate, 8-(4-chlorophenylthio)-, Rp isomer (Rp-8-pCPT-cGMPS), adenosine 3′,5′-cyclic monophosphorothioate, Rp isomer (Rp-cAMPS), 1-H[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), L-thiocitrulline, N^(G)-nitro-L-arginine methyl ester hydochloride (L-NAME), protein kinase A inhibitor 6-22 amide, and PKG substrate BPDEtide were purchased from Calbiochem (La Jolla, Calif.). Peptides A, C, and D, and MAPK p42 protein were obtained from Santa Cruz Biotechnology (Santa Cruz, Calif.). The following antibodies were obtained and used according to the manufacturer's instructions: anti-phospho-VASP (Ser239) from UpstateCell Signaling Solutions (Lake Placid, N.Y.), polyclonal antibodies to phosphorylated MAPK (Ab^(pTpYmapk)) and phospho-Elk1 (Ser³⁸³; Ab^(pElk1)) from Cell Signaling Technology (Beverly, Mass.), and anti-pY MAPK (Ab^(pYmapk)), anti-pT MAPK (Ab^(pTmapk)), and α-actin from Sigma (St. Louis, Mo.).

6.2 Results

Aplysia SNs Contain a Neuronal Type-I PKG.

To investigate the role of the PKG pathway in the induction of LTH, an Aplysia PKG was cloned (GenBank accession number AY362340). ApPKG cDNA contains an open reading frame encoding a putative 733 aa protein. In concordance with all of the known PKGs, the Aplysia kinase contains two tandem cyclic nucleotide-binding domains and a C-terminal catalytic domain (FIG. 1A, top). The predicted protein encoded by the apPKG sequence is highly similar to known cGMP-dependent protein kinases with greater than 50% amino acid identity to Drosophila PKGs and to the mammalian type-I and -II PKG isoforms (FIG. 1A, bottom). However, it appears to be most closely related to Drosophila DG1 (FIG. 1B).

To determine the pattern of apPKG expression, a Northern blot of total RNA from various tissues from the adult animals was probed with a ³²P-labeled 283 bp fragment corresponding to bases 209-492 of the cDNA. The probe detected a single 3.0 kb transcript that was expressed in ganglia, but not in muscle or the genital organs (FIG. 1C). A ³²P-labeled probe to 5S ribosomal RNA detected a 0.19 kb transcript in all of the tissues (FIG. 1C).

To localize the source of the message, the apPKG probe was used to generate sense and antisense riboprobes for in situ hybridization. The antisense probe detected high levels of apPKG mRNA expression in the SN cluster and in most of the neurons in the pedal and pleural ganglia (FIG. 1D, left panel). Negligible labeling resulted from the sense probe (FIG. 1D, right panel).

The catalytic properties of apPKG were then investigated by first expressing apPKG cDNA in a baculovirus/Sf9 system. An inactive His-tagged recombinant apPKG was produced when the cells were grown in the presence of serum, and a constitutively active apPKG was made when the cells were deprived of serum. Both recombinant apPKG forms were purified via affinity chromatography.

Inactive apPKG was activated by 100 nM 8-Br-cGMP and readily phosphorylated BPDEtide, a peptide substrate for all of the type-I PKGs (Glass and Krebs, 1982) (FIG. 2A). Comparison of active apPKG with recombinant bovine type-Iα soluble PKG showed that both kinases readily phosphorylated several PKG peptide substrates (FIG. 2B). Significantly, neither kinase phosphorylated peptide C, which is the preferred substrate for membrane-bound type-II PKG (Hall et al., 1999) (FIG. 2B). The protein VASP, whose serine at position 239 is recognized specifically by type-I soluble PKGs (Smolenski et al., 1998), was also examined, and it was found that both the bovine and Aplysia kinases phosphorylated this site (FIG. 2C). These studies establish apPKG as a member of the soluble type-I family of PKGs.

ApPKG is Located in Axons where it is Activated by Nerve Injury and Retrogradely Transported to the Cell Body of the SNs.

Aplysia peripheral nerves p5-p9 innervate the mid-body wall and tail region and contain axons of the SNs (Walters et al., 1983a,b; Billy and Walters, 1989). To determine whether apPKG is in axons, a rabbit polyclonal antibody, Ab^(apPKG), was raised against a peptide located at the N terminus (amino acids 18-128) of the protein. Ab^(apPKG) was affinity purified and used to probe Western blots. Ab^(apPKG) recognized 80 and 100 kDa polypeptides in pedal and pleural ganglion extracts, but not in muscle or genital tissue as well as the affinity-purified 80 kDa recombinant protein (FIG. 3A). It is believed that the 100 kDa band is the dominant form of apPKG, because it is most abundant and has a greater affinity for cGMP than the 80 kDa constituent. Both bands were also recognized by a commercial antibody that was generated against human type-IαPKG (amino acid residues 657-671: 50% identity to apPKG). apPKG contains consensus sequences for several kinases and other enzymes and the 100 kDa constituent might contain one or more posttranslational modifications.

To determine whether apPKG is present in axons, axoplasm was extruded from nerves p5-p9. FIG. 3A shows that both polypeptides were present. The SNs have axons in nerves p5-p9, and immunocytochemistry with AbapPKG stained essentially all of the SNs in the cluster (FIG. 3B). To determine the effects of nerve injury on axonal apPKG, nerves p5-p9 on one side were crushed, thereby axotomizing the axons in each nerve. At various times afterward, apPKG activity was assayed in the ipsilateral and contralateral pedal and pleural ganglia, which included the cluster of SN cell bodies. As shown in FIG. 4A, there was a delay of about 16 hr after nerve crush before significant apPKG activity appeared in the ipsilateral pleural ganglion. The activity then increased for at least 24 hr, but was not significantly different from baseline at 48 hr. Little or no apPKG activity was detected in the contralateral control pleural ganglion (FIG. 4A). Significantly, apPKG activity in the cell bodies of the ipsilateral pedal ganglion neurons remained at basal level during the 48 hr after axotomy (FIG. 4A), indicating that the activation of apPKG is selective for neurons in the pleural ganglion.

The long delay before apPKG activity appeared in the cell bodies is consistent with the idea that the kinase is a positive injury signal. Moreover, apPKG contains a nuclear localization sequence (NLS) that can provide access to the retrograde transport system (Ambron and Walters, 1996; Schmied and Ambron, 1997; Hanz et al., 2003). A standard crush-ligation protocol was used to investigate this possibility (Ambron et al., 1995; Johanson et al., 1995) (FIG. 4B). Nerves p5-p9 were crushed unilaterally, and a ligation was placed on each nerve proximal to the crush site. Proteins that are transported away from the crush site (toward the cell bodies) accumulate in the axoplasm behind the ligation where they can be collected. 24 hr later 0.5 cm nerve segments were removed as follows: (1) proximal to the crush (Cr) site, (2) distal to the ligation on the crushed nerves (the Cr/Lig site), and (3) distal to the ligation on the control side (the Lig site) (FIG. 4B, top). Axoplasm was then extruded from each segment, as well as from segments of nerves p5-p9 from an animal that did not receive a nerve injury. When equal amounts of each axoplasm were screened for apPKG activity, there was a 10-fold increase in apPKG activity in axoplasm from the Cr/Lig segment relative to basal activity in axoplasm from noninjured animals (FIG. 4B, bottom). In contrast, apPKG activity at the Cr segment was only threefold greater than the basal activity, and that at the Lig segment was at the basal level.

The accumulation of apPKG activity at the Cr/Lig site was supported by immunocytochemical studies in which Ab^(apPKG) was used to examine the distribution of ap-PKG protein in noninjured and injured nerves. apPKG staining was uniformly distributed along axons in naive nerves (FIG. 4C, panel 1). After nerve crush, however, there was a decrease in the staining at the Cr site (FIG. 4C, panel 3) and a significant increase at the terminations of axons at the Cr/Lig site (panel 4). There was no increase in staining at the Lig site (FIG. 4C, panel 2). The accumulation of both apPKG protein and activity at the Cr/Lig site relative to the Cr and Lig sites is strong evidence that apPKG is a positive injury signal (Ambron et al., 1995).

The Nitric Oxide-cGMP-PKG Pathway Regulates the Induction of LTH in SNs In Vitro.

When Aplysia SNs are axotomized by removal from the ganglion and individually placed in culture, they regenerate their axons and exhibit a decrease in spike threshold and an increase in repetitive firing, which are two characteristics of LTH (Ambron et al., 1996; Bedi et al., 1998; Sung and Ambron, 2004). To examine the appearance of this LTH in greater detail, the electrical properties from the SNs in vitro were recorded on the second to the seventh day and compared to the electrical recordings from SNs within the cluster in situ. Approximately 10% of the cells tested in vitro did not have a sufficient resting potential or were refractory to firing and were not included in the study. The same decrease was detected in threshold and increase in repetitive firing as reported previously, and it was also found that the axotomized SNs exhibited significant spike broadening and an increase in spike amplitude relative to the controls (FIG. 5). The changes were significant on the second day and persisted until at least the seventh day. The fact that these same four changes in electrical properties occur in hyperexcitable rat DRG SNs after axotomy (Abdulla and Smith, 2001), affirms the use of dissociated Aplysia SNs as a model system for studies of sensory alterations that may contribute to neuropathic pain.

To investigate whether the appearance of the LTH requires apPKG activity, the SNs were removed from the ganglion in the presence of either the soluble guanylyl cyclase (sGC) inhibitor ODQ, the PKG inhibitor Rp-8-pCPT-cGMPS, or the PKA inhibitor Rp-cAMPS. The latter was used because PKA has properties in common with PKG (Scott, 1991; Francis and Corbin, 1994) and has been implicated in various forms of synaptic plasticity in Aplysia SNs (Ghirardi et al., 1992; Byrne and Kandel, 1996; Bedi et al., 1998; Muller and Carew, 1998; Chain et al., 1999; Liao et al., 1999; Sutton and Carew, 2000; Antonov et al., 2003). The cells were subsequently allowed to regenerate in the presence of the inhibitors in vitro, and on the third day, their electrophysiological properties were compared with those of SNs that were removed from the same ganglia at the same time, but that had not been exposed to a drug. It was found that both Rp-8-pCPT-cGMPS and ODQ prevented the increase in repetitive firing (FIG. 6A) and the decrease in spike threshold (B). In contrast, the PKA inhibitor had no significant effect on either parameter. Neither Rp-8-pCPT-cGMPS nor ODQ inhibited the appearance of excitability when added to 2-d-cultured SN, confirming the data in FIG. 4A showing that apPKG activation is transient. Neither spike broadening nor amplitude were evaluated in these experiments.

None of these treatments, which used similar or even lower concentrations of the drugs than that reported for mammalian (Monfort et al., 2002) and Aplysia neurons (Lewin and Walters, 1999), affected the resting membrane potential. Significantly, they also did not alter the extent or pattern of neurite growth. In contrast, exposing SNs to U0126, a selective MEK inhibitor, produced severe growth defects.

The inhibition of LTH by the sGC and PKG inhibitors indicated that cGMP synthesis and PKG activation occur within the sensory neurons. NO is known to elevate cGMP production via sGC (Schlossmann et al., 2003), and NO is produced by the enzyme NOS (Bredt and Snyder, 1990; Moroz et al., 1996). To investigate the likelihood that NOS activation is required for LTH, RT-PCR was used to detect the cellular levels of apn-NOS mRNA in single SNs immediately after their removal from the ganglion (0 hr) or after 16 hr in vitro. As shown in FIG. 6C (top panel), four of five cells expressed significant amounts of apnNOS mRNA after 16 hr in vitro, whereas none was detected in the 0 hr cells. In contrast, the mRNA for the SN-specific neuropeptide sensorin A (Brunet et al., 1991) and the neuronspecific isoform of actin (DesGroseillers et al., 1994) was abundant in every cell (FIG. 6C, second and third panels, respectively).

L-Thiocitrulline is an effective inhibitor of apNOS, and it was found that exposing the SNs to this drug in vitro as above markedly reduced the maximum firing relative to untreated controls (FIG. 6D). Another NOS inhibitor, L-NAME also reduced LTH under the same conditions, but was not as effective as L-thiocitrulline (FIG. 6D). These data indicate that the NO-cGMP-PKG pathway is required for inducing LTH in the SNs.

Somatic apMAPK is Phosphorylated at its Activation Site by apPKG.

The induction of LTH in Aplysia SNs after peripheral injury requires gene transcription (Lewin and Walters, 1999) and could be effected directly by apPKG if the kinase translocated to the nucleus after entering the cell body. Immunostaining revealed a low constitutive level of apPKG in some SN nuclei, but there was no increase after axotomy (FIG. 3B). This suggested that apPKG contributes to the induction of LTH by activating a factor that is imported into the nucleus. An ERK member of the MAPK family is a good candidate for this factor, because LTH can be induced by injecting recombinant ERK1 into SN somata (Sung et al., 2001). To assess whether an ERK was activated after injury, p5-p9 were crushed in vivo and used to produce Western blots of injured and control pleural neurons, which were probed with an antibody (Ab^(pTpYmapk)) that recognizes ERKs that have been activated by dual phosphorylation at the T-E-Y site. The antibody recognized a single 43 kDa polypeptide that had little activity in pleural neurons 4 or 8 hr after the injury (FIG. 7A, top panel). By 16 hr, however, there was more active kinase on the crush side relative to the contralateral control, and the level continued to increase for at least 48 hr (FIG. 7A, top panel).

Aplysia neurons contain an ERK2 homolog, apMAPK (Michael et al., 1998), that enters the nucleus after injury in vitro (Martin et al., 1997). apMAPK has the same catalytic domain and T-E-Y activation site as ERKs 1 and 2. To determine whether apMAPK was the kinase activated after injury, we probed the same blot with an antibody, D8, which specifically recognizes apMAPK (Martin et al., 1997), and found that the antibody recognized the 43 kDa injury-activated kinase (FIG. 7A, bottom panel). The antibody detects both active and inactive apMAPK, and there was little difference in the amount of total apMAPK protein among the samples. The finding that apMAPK activity began to increase just after the arrival of active apPKG in the cell body (compare FIGS. 4A, 7A) suggested a link between apPKG and apMAPK. This idea was tested by preparing a lysate of pleural ganglion neurons and using Ab^(pTpYmapk) to monitor samples for active apMAPK. Little endogenous phospho-apMAPK was detected in the lysate (FIG. 7B, lane 1), but adding active apPKG or 8-Br-cGMP markedly enhanced the level of active ap-MAPK (lanes 2, 3) that was recognized by antibody D8. Surprisingly, the activation was not blocked by U0126, a potent inhibitor of the upstream kinase MEK (FIG. 7B, lane 4). Interestingly, adding active apPKG to axoplasm did not activate apMAPK (FIG. 7B, lanes 5, 6) (see Discussion). Another study with the lysate showed that the apPKG-activated ap-MAPK phosphorylated its physiological substrate, Elk1, at the appropriate Ser³⁸³ (FIG. 7C, lane 3).

The activation of apMAPK in the presence of U0126 suggested that apPKG activates apMAPK directly, and therefore active apPKG was incubated with recombinant mammalian ERK2. ERK2 was a surrogate for apMAPK. Both kinases contain the target T-E-Y site, however. As in the lysate, apPKG activated ERK2 to phosphorylate Elk1 (FIG. 7C, lane 6). When the experiment was repeated, the activated ERK2 was recognized by Ab^(pTpYMAPK), indicating that it was doubly phosphorylated (FIG. 7D, top, lane 2). The activation was specific, because it was reduced in the presence of BPDEtide (FIG. 7D, top, lane 3). ERK2 is maximally activated when both the -T- and -Y- amino acids are phosphorylated, yet PKGs are serine/threonine kinases. It is relevant, therefore, that bacterial recombinant ERK2 has a low level of activity that is attributable to the presence of a phosphate on the -Y- moiety (Cha and Shapiro, 2001). Indeed, the antibody to ERKpY185 recognized the recombinant ERK2 substrate on a Western blot (FIG. 7D, middle). There was no increase in phosphorylation of the -Y- when ERK2 was incubated with apPKG (FIG. 7D, middle, lane 2). However, when a duplicate blot was probed with an antibody to ERKpT183, there was an increase in the phosphorylation of the -T- in the presence of apPKG (FIG. 7D, bottom, lane 2). This antibody reacts specifically with the monophosphorylated threonine and doubly phosphorylated ERKs. This result indicates that ap-PKG can fully activate ERK2 by phosphorylating ERK2 that already contains a phosphate on the Y-185.

It was next examined whether incubating apPKG and recombinant ERK2 produces an active ERK2 with enzymatic activity comparable with that produced by MEK1, which produces the maximally activated kinase. ERK2 activity was measured by the phosphorylation of Elk1 at Ser³⁸³, and MEK1 was activated by the catalytic subunit of MEKK1 (Xu et al., 1995). As anticipated, apPKG produced an ERK2 with similar activity as that produced by MEK1 (FIG. 7E). This result indicates that, in addition to MEK1, PKG is an activator of ERK2.

Because PKG phosphorylates T183 on ERK2 that already has a phosphate on Y185 (FIG. 7D), an essential question is whether monophosphorylated apMAPK at -Y- is available for phosphorylation by apPKG after axotomy. This question was answered by first crushing p5-p9 in vivo. Samples were then blotted and probed with an antibody to ERKpY185. Indeed, apMAPKpY was present in pleural neurons 8 hr after the injury, and its expression was increased on the crush side relative to the contralateral control at 16 hr (FIG. 7F, top panel). ApMAPKpY was not detected in two naive animals that were analyzed (FIG. 7F, top panel). Probing the blot with an antibody that detects both active and inactive apMAPK showed that there was little difference in the amount of total apMAPK protein among the samples (FIG. 7F, bottom panel). Thus, the presence of apMAPKpY in the soma 16 hr after the injury, in conjunction with the arrival of apPKG from the crush site (FIG. 4A), should result in full activation of apMAPK.

The Level of apMAPK in the Nucleus of SNs In Vitro is Reduced when apPKG Activity is Inhibited.

The evidence above indicates that apPKG does not enter the nucleus of the SNs in response to nerve crush in vivo (FIG. 3B). Nevertheless, type-I PKGs have a putative NLS (Gudi et al., 1997), and apPKG has a short stretch of positively charged amino acids (453KCLKKKHI) in the ATP-binding domain that could function as an NLS. The import of proteins into the nucleus of Aplysia neurons is readily assessed by injecting their fluorescently labeled cognates directly into the cell body (Ambron et al., 1992; Schmied et al., 1993; Gunstream et al., 1995). Therefore Alexa labeled recombinant apPKG was injected into the soma of SNs after 2 d in vitro, the neurites were severed to initiate an injury response, and, 30 min later, the cells were examined by fluorescence microscopy. All of the labeled protein remained in the cytoplasm (FIG. 8A). Alexa labeled BSA also remained in the cytoplasm after injection, as expected (Ambron et al., 1992; Schmied et al., 1993; Gunstream et al., 1995). In contrast, injected Alexa-labeled active recombinant vertebrate ERK1, which is imported into the nucleus in a variety of cell types (Karin, 1994) rapidly entered the nucleus where it was distributed in discrete patches (FIG. 8A). Similar patches have been seen after the import of other proteins into Aplysia nuclei (Ambron et al., 1992; Schmied et al., 1993; Gunstream et al., 1995).

The inability of apPKG to enter the nucleus of Aplysia SNs is consistent with the idea that it contributes to the induction of LTH by promoting the nuclear import of apMAPK. If so, then inhibiting the apPKG pathway should block the axotomy-induced entry of apMAPK into the nucleus. Therefore, SNs were exposed in vitro either to the PKG blocker, Rp-8-pCPTcGMPS, or the PKA blocker, Rp-cAMPS, under the conditions that induce LTH (FIG. 6A). On the third day, the cells were fixed, permeabilized, and exposed to antibody D8 to visualize ap-MAPK. Confocal microscopy showed that untreated cells and those exposed to Rp-cAMPS had the same levels of nuclear staining (FIG. 8B). In contrast, there was a dramatic reduction in nuclear staining in the cells treated with Rp-8-pCPT-cGMPS (FIG. 8B). Thus, both the induction of LTH and the presence of ap-MAPK in the nucleus depend on apPKG activity.

6.3 Discussion

Persistent neuropathic pain in humans after nerve injury is physically and psychologically debilitating. Because an important component of this pain is often the LTH that appears in primary afferent neurons, it is important to understand how axotomy induces LTH. An LTH with similar properties appears in nociceptive SNs of Aplysia after axotomy, and the experimental advantages of these neurons were exploited to define a signaling pathway responsible for the induction of LTH. One major surprise was that this pathway involves the phosphorylation of apMAPK by PKG. This is an alternative to the traditional MAP kinase cascade and suggests that PKG and MAPK have unique roles after nerve injury.

ApPKG is a Positive Injury Signal in SNs.

An Aplysia type-I PKG was cloned whose mRNA is in the SNs and other neurons in the pleural ganglion (FIG. 1D). When the peripheral nerves were crushed in vivo, apPKG activity appeared in the somata of pleural neurons, but only after a delay of about 16 hr (FIG. 4A). A similar delay in response to nerve injury has been attributed to positive injury signals (Schmied et al., 1993; Sung et al., 2001; Lin et al., 2003). Western blots probed with Ab^(apPKG) showed that apPKG is present in axoplasm extruded from the peripheral nerves (FIG. 3A). In addition, because proteins are retrogradely transported along Aplysia axons at a rate of 1.5 mm/hr (Ambron et al., 1992; Schmied et al., 1993; Gunstream et al., 1995), the delay is consistent with the transport of apPKG from the crush site, which was located 2 cm from the ganglion. When a ligation was placed proximal to the crush site it was found that activated apPKG and total apPKG protein accumulated on the distal side of the ligation, relative to the crush site (FIG. 4B,C). These findings establish apPKG as a positive injury signal.

The mechanism responsible for the injury-induced transport is not known. Although apPKG contains several potential myristoylation sites that could bind it to vesicles after injury, apPKG was found to be soluble after subcellular fractionation. Hanz et al. (2003) have shown that proteins containing an NLS are retrogradely transported via dynein and importins in the rat sciatic nerve, and apPKG may be transported using a similar mechanism.

The Appearance of the LTH in Axotomized Aplysia SNs In Vitro Depends on NOS, sGC, apPKG, and apMAPK.

The LTH that appears in both the SNs (FIG. 5) and mammalian nociceptive SNs after nerve injury (Abdulla and Smith, 2001) have similar electrophysiological properties. If this congruence reflects conserved mechanisms, then LTH might be induced by common pathways in both types of cells. SNs were examined in vitro and it was found that the axotomy-induced reduction in spike threshold and the increase in hyperexcitability were blocked by Rp-8-pCPT-cGMPS, ODQ, and L-thiocitrulline, inhibitors of apPKG, sGC, and NOS, respectively (FIG. 6A,B,D). Moreover, Rp-8-pCPT-cGMPS also caused a nearly threefold reduction in the level of apMAPK in the nucleus of the SNs (FIG. 8B). These findings pointed to a direct relationship between the activation of NOS and apPKG, the entry of apMAPK into the nucleus, and the induction of LTH. The finding that NOS mRNA expression in the SNs was increased after injury (FIG. 6C) implies that the level of NOS protein might be the rate-limiting step in the pathway. Neuronal NOS (nNOS) mRNA and protein expression also increase in DRG neurons after severing their peripheral axons (Verge et al., 1992; Fiallos-Estrada et al., 1993; Zhang et al., 1993).

cAMP and PKA have also been implicated in the induction and maintenance of LTH (Scholz and Byrne, 1988; Goldsmith and Abrams, 1992; Bedi et al., 1998), and this was a concern here because PKA and PKG have properties in common. However, it was found that exposing the SNs to Rp-cAMPS, a membrane-permeable inhibitor of PKA, neither prevented the induction of LTH nor blocked apMAPK entry into the nucleus. This confirms previous findings that PKA inhibitors failed to block LTH induced by noxious stimulation (Lewin and Walters, 1999).

The activation of both apPKG and ap-MAPK by injury was intriguing, because it suggested a possible link between these two kinases (Zaragoza et al., 2002). Indeed, when active apPKG was added to a neuronal lysate, it not only activated ap-MAPK (FIG. 7B), but did so via a pathway that did not require MEK, the ubiquitous upstream activator of the ERKs. This response was physiologically relevant, because activated apMAPK phosphorylated its nuclear substrate Elk1 at the Ser383 (FIG. 7C), which is essential for transcriptional activity (Marais et al., 1993; Whitmarsh et al., 1995). A direct interaction was established between the kinases when active apPKG was incubated with recombinant vertebrate ERK2, which has the same T-E-Y activation site as apMAPK. It was found that the ERK2 was both activated (FIG. 7C) and doubly phosphorylated (D, top). The latter finding indicated that the ERK2 was fully active, and this was supported by another study in which equal amounts of ap-PKG or MEK1 activated ERK2 to comparable levels (FIG. 7E).

The indications that apPKG produces a maximally activated ERK2 creates a paradox, because PKGs are serine-threonine kinases that cannot phosphorylate the tyrosine. However, because the recombinant ERK2 already contains phospho-tyrosine-185 (FIG. 7D, middle), the phosphorylation of threonine-183 by apPKG (FIG. 7D, bottom) should produce a fully activated ERK2. The phosphorylation of apMAPKpY by apPKG is attractive, because ERK2pY185 has been detected in vertebrate cells (Yao et al., 2000; Cha and Shapiro, 2001; Zhou and Zhang, 2002), and the evidence indicates that apMAPKpY expression increases in Aplysia neurons after nerve injury (FIG. 7F). apMAPKpY could be produced by a phosphatase that removes the phosphate from doubly phosphorylated apMAPK, or by an injury-activated tyrosine kinase that phosphorylates the tyrosine at the T-E-Y site. The possibility that the induction of LTH requires the convergence of apPKG and a tyrosine kinase on apMAPK would confer more control over this pathway. This is reasonable given that LTH radically alters the function of the injured neurons and leads to significant changes in the behavior of the animal.

apMAPK is also present in axoplasm, but is not activated by nerve injury (Sung et al., 2001; Lin et al., 2003), which is paradoxical, given that injury activates the axoplasmic apPKG. One explanation would be that the two kinases are located in different axons. However, when active apPKG was added to axoplasm extruded from the peripheral nerves, apMAPK was not activated under the same conditions that caused its activation in the cell soma (FIG. 7B). Evidently, there is a mechanism in the axon that shields apMAPK from apPKG. Nevertheless, these observations mean that, after its activation in the axon by injury, apPKG must be transported back to the soma to influence nuclear events via apMAPK.

With regard to the molecular pathway described herein, the possibility remains that nerve injury may cause translocation of apMAPK or activation of somatic PKG by other pathways as well. An NO-cGMP-PKG-MAPK signaling pathway might also be important for LTH induced by the inflammation that develops around a nerve injury site, both in mammals (Millan, 1999; Zimmermann, 2001) and in Aplysia (Clatworthy et al., 1995; Clatworthy and Grose, 1999; Farr et al., 1999, 2001). How might nuclear apMAPK regulate LTH? apMAPK can phosphorylate CREB2 (cAMP response element-binding protein 2), a cAMP response element (CRE) site repressor (Bartsch et al., 1995; Michael et al., 1998), C/EBP (CCAAT/enhancer-binding protein), a transcription factor that binds to the estrogen response element (ERE) site (Alberini et al., 1994), and Elk1, a transcription factor that regulates the serum response element (SRE) site (Lin et al., 2003). Binding to all three sites increases after nerve injury, but with different time courses. Thus, binding of C/EBP and CREB to the ERE and CRE, respectively, is rapid, but relatively short lived (Dash et al., 1998; Sung et al., 2001), whereas the binding of Elk1 to the SRE is biphasic, with an early phase that lasts for a few hours and a second phase that persists for weeks (Lin et al., 2003). CREB is phosphorylated in DRG neurons in response to intense activity (Ji and Woolf, 2001), and the CRE site is required for the LTH response to noxious stimuli in Aplysia SNs (Lewin and Walters, 1999). These considerations point to CREB2 and C/EBP as targets of apMAPK during the initial induction of the LTH in the SNs. The persistence of LTH for weeks may be mediated by the phosphorylation of Elk1 by apMAPK. This pathway is selective, because inhibiting NOS, sGC, or PKG prevented the appearance of LTH in dissociated SNs, but did not block growth. If the link between this pathway and LTH represents a fundamental, widely conserved relationship, then therapeutic interventions that target this pathway may be used to mitigate persistent pain after nerve injury without blocking axon regeneration.

7. EXAMPLE Blocking of PKG Activity in Rat

FIG. 9 shows that the activation of PKG can be blocked at the site of injury using an inhibitor of PKG, Rp-8-pCPT-cGMPS (“RP-G”). The experiment was carried out as follows. A rat was anesthetized using a standard protocol and the sciatic nerve was exposed and injured in either the absence or the presence of RP-G, or an inhibitor of PKA, Rp-8-pCPT-cAMPS, (“RP-A”). Four hours later, the nerve was sectioned as shown in the schematic above the graph in FIG. 9, and each segment was assayed for PKG activity. In the absence of inhibitor (black bars), active PKG had been transported toward the cell bodies and was enriched in P3. RP-A (gray bars) did not change this pattern. In contrast, RP-G (white bars) abolished the activity.

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Various publications are cited herein, the contents of which are hereby incorporated by reference in their entireties. 

1. A method of treating chronic pain in a human subject suffering from peripheral nervous system hyperalgesia, comprising administering, to the subject, an effective amount of an RNAi that decreases the level of human protein kinase G-1 alpha (type-I alpha PKG) protein in the cell body of a sensory neuron in the peripheral nervous system of the subject, wherein the RNAi is administered to the location from which the pain arises, and wherein the human protein kinase G-1 alpha (type-I alpha PKG) amino acid sequence is as set forth in SEQ ID NO:54.
 2. A method of treating chronic pain in a human subject suffering from peripheral nervous system hyperalgesia, comprising administering, to the subject, an effective amount of an agent that decreases the level of human protein kinase G protein in the cell body of a sensory neuron in the peripheral nervous system of the subject, wherein the agent is administered to the location from which the pain arises, wherein the agent is an antisense nucleic acid molecule or RNAi that inhibits expression of human protein kinase G, and wherein the human protein kinase G amino acid sequence is as set forth in SEQ ID NO:54.
 3. A method of inhibiting long-term hyperexcitability in the cell body of a sensory neuron in the peripheral nervous system of a human subject, comprising administering, to the subject, an effective amount of an agent that decreases the level of human protein kinase G protein in the cell body of the sensory neuron in the peripheral nervous system of the subject, wherein the agent is an RNAi that inhibits expression of human protein kinase G, and wherein the human protein kinase G amino acid sequence is as set forth in SEQ ID NO:54.
 4. A method of inhibiting long-term hyperexcitability in the cell body of a sensory neuron in the peripheral nervous system of a human subject, comprising administering, to the subject, an effective amount of an agent that decreases the level of human protein kinase G protein in the cell body of the sensory neuron in the peripheral nervous system of the subject, wherein the agent is an antisense nucleic acid molecule that inhibits expression of human protein kinase G, and wherein the human protein kinase G amino acid sequence is as set forth in SEQ ID NO:54.
 5. The method of any one of claims 1, 2, 3, or 4, wherein the sensory neuron is a human sensory neuron.
 6. The method of claim 1, wherein the RNAi is administered topically.
 7. The method of claim 1, wherein the RNAi is administered by local injection.
 8. The method of claim 2, 3, or 4, wherein the agent is administered topically.
 9. The method of claim 2, 3, or 4, wherein the agent is administered by local injection. 